1. Field of the Invention
This invention relates generally to a process for warm digestion of sludge, carried out under aerobic and anaerobic conditions.
2. Description of the Prior Art
With continued growth of industry and population, the problems associated with wastewater disposal are correspondingly increased. Although physical, chemical and biological treatment systems have been developed which can efficiently treat polluted waters to produce an effluent suitable for release to natural receiving water, almost all of the basic wastewater treatment systems currently in use, including clarification, chemical precipitation, biological filtration and activated sludge, convert the water pollutants into a concentrated form called sludge, Particularly in the activated sludge process, which is among the most popular of conventionally employed wastewater treatment systems, there is usually a significant net positive production of volatile suspended solids (MLVSS), i.e., the rate of cell synthesis exceeds the rate of cell destruction. Therefore, an increasing inventory of sludge builds up and the excess activated sludge must be discarded from the process continuously or periodically.
As the overall volumes of wastewater requiring treatment increases, particularly under the impetus of increasingly stringest pollution control legislation, the quantity of waste sludge produced by the above-mentioned wastewater treatment processes is correspondingly increased. Accordingly, it is highly desirable to process this waste sludge in such manner that it can be readily and economically disposed of without creating further pollution of the ecosphere. While much effort has been spent in development of improvements in sludge treatment technology as well as in refinement of existing sludge treatment processes, there still exists a great need for better and more efficient sludge treatment systems.
The basic aim of all sludge treatment processes is to economically and efficiently reduce and stabilize sludge solids. In addition, the sludge treatment system should desirably also produce an end product which is fully suitable for final disposal without further physical or chemical treatment. In conventional practice sludge disposal is commonly carried out by either ocean dumping, combustion, land filling or land spreading. In many instances, land disposal is employed and is particularly attractive due to minimal long-term environmental effects. In fact, land spreading of sludge may be highly advantageous in promoting reconditioning of the soil. However, the use of land spreading as a final sludge disposal method requires a well-pasteurized end product, so that the concentration of pathogenic organisms in the sludge is sufficiently low to avoid a potential health hazard in disposition of the sludge.
Traditionally, three distinct processes have been widely utilized for treating waste sludge: oxidation ponds, anaerobic digestion and aerobic digestion.
Oxidation ponds are generally employed in the form of comparatively shallow excavated basins in the earth which extend over an area of land and retain wastewater prior to its final disposal. Such ponds permit the biological oxidation of organic material by natural or artificially accelerated transfer of oxygen to the water from the ambient air. During the bio-oxidation process, the solids in the wastewater are biologically degraded to some extent and ultimately settle to the bottom of the pond, where they may become anaerobic and be further stabilized. Periodically the pond may be drained and the settled sludge dredged out to renew the volumetric capacity of the pond for further wastewater treatment, and the withdrawn sludge is utilized for example for landfill. Oxidation ponds thus represent a functionally simple system for wastewater and sludge treatment. The use of oxidation ponds, however, has limited utility, since their operation requires sizable land areas. Moreover, no significant reduction of the level of pathogens in the sludge is accomplished by this treatment and disposal method.
Anaerobic digestion has generally been the most extensively used digestion process for stabilizing concentrated organic solids, such as are removed from settling tanks, biological filters and activated sludge plants. In common practice, the excess sludge is accumulated in large domed digesters where the sludge is fermented anaerobically for 20-30 days. The major reasons for commercial acceptance of anerobic sludge digestion are that this method is capable of stabilizing large volumes of dilute organic slurries, results in low biological solids (biomass) production, produces a relatively easily dewaterable sludge and is a producer of methane gas. Additionally, it has been variously alleged that anaerobic digestion produces a pasteurized sludge. Even though this pasteurizing capability of anaerobic digestion is questionable, anaerobic digestion is widely used in practice because it reduces the solid residue to a reasonably stable form which can be discarded as land fill without creating a substantial nuisance. The anaerobic digestion is characteristically carried out in large scale tanks which are more or less thoroughly mixed, either by mechanical means or by the recycling of compressed digester gas. Such mixing rapidly increases the sludge stabilization reactions, by creating a large zone of active decomposition.
As indicated above, anaerobic digestion has commonly been practiced with long retention times on the order of 20 - 30 days, without any heat input to the system. It has been found by the prior art that elevated temperatures in the mesophilic range of 30.degree. to 40.degree. C. facilitate reduction of the retention time requirement, to about 12 - 20 days. Such reduction in treatment time is a consequence of the fact that the rate of activity of the organisms responsible for digestion is greatly influenced by temperature, and that in the 30.degree. to 40.degree. C. temperature range highly active mesophilic microorganisms are the dominant microbial strain in the sludge undergoing digestion. The best temperatures for mesophilic digestion are in the range of about 35.degree. to 38.degree. C., with minimum retention times on the order of 12 - 15 days. Temperatures up to 35.degree. C. increase the rate of digestion and may allow shorter retention times, but at the expense of system operating stability while temperatures below 35.degree. C. require longer retention times.
Methane gas is produced during anaerobic digestion and is characteristically used in combustion heaters to offset heat losses of the anaerobic digestion system operating at elevated temperature. However, seasonal temperature variations and fluctuations in the suspended solids level of the influent sludge have a significant effect on both the methane gas production and the amount of heating which is necessary to maintain the digestion zone at the desired elevated temperature operating level. As a result, if elevated temperature conditions are to be maintained year round in the anaerobic digestion zone, an auxiliary heat source is generally an essential apparatus element of the sludge digestion system.
Since the rates of anaerobic digestion and resultant methane gas formation are strongly influenced by the suspended solids content of the sludge undergoing treatment and by the temperature level in the digestion zone, it is in general desirable to feed as concentrated a sludge as possible to the digester, thereby minimizing heat losses in the effluent stabilized sludge stream discharged from the anaerobic digester while maximizing methane production in the digester. However, even with such provisions elevated temperatures are difficult to maintain economically in the anaerobic digestion zone, especially during winter months. Furthermore, even comparatively small temperature fluctuations in the anaerobic digestion zone may result in disproportionately severe process upset and souring of the digester contents, as is well known.
In the anaerobic digestion process, the sludge solids being treated undergo essentially three distinct sequential treatment phases: first, a period of solubilization, secondly, a period of intensive acid production (acidification), and finally, a period of intensive digestion and stabilization (gasification). Each of these steps is characterized by the production of various intermediate and end products in the digestion zone. Under normal operating conditions, all three phases occur simultaneously. The primary gases produced during the final gasification phase are methane and carbon dioxide, which normally form more than 95% of the gas evolved, with 65-70% comprising methane. Production of methane gas in anaerobic digestion results from the breakdown of many compounds by numerous interdependent biochemical reactions which take place in an orderly and integrated fashion. The complex organic species in the sludge are converted by a variety of common bacteria called acid-formers to volatile acids and alcohols, without production of methane. These products from the acid-forming phase are then converted to methane gas by another variety of bacteria known as methane-formers.
The facultative acid-forming bacteria utilized in anaerobic digestion are hardy and highly resistant to process changes in their environment. Methane-forming bacteria, on the other hand, require anaerobic conditions and are extremely sensitive to process changes in their environment. For such reasons, oxygen should not be present in the anaerobic digestion zone. The inadvertent introduction of air to the digester will adversely affect methane fermentation, as well as creating a potentially hazardous situation due to combination of the combustible methane gas with oxygen. In addition, methane-forming bacteria are sensitive to such process conditions to pH variations and presence of detergents, ammonia and sulfides. In this respect, temperature stability of the anaerobic digestion zone is particulary important. The methane-formers necessary in the digestion process are highly susceptible to temperature fluctuations, which decrease their activity and viability, resulting in excessive relative growth of acid-formers. This in turn results in inadequately stablized sludge and a sludge product which is unsuitable, without further treatment, for landfill or similar disposal. Further, these methane-formers have a relatively low rate of growth and such factor necessitates the long retention times employed for anaerobic digestion even at mesophilic temperatures. Due to this low growth rate, there is danger of washing the methane-forming organisms out of the digester if the sludge solids retention time therein is reduced beyond the previously described retention time lower limits. Inasmuch as the anaerobic digester thus requires long retention times to insure the presence of adequate methane-formers and the influent sludge flow rate to the digestion zone is in general quite low, the tankage requirements for the digester are very large. Operation at elevated temperature is thus difficult, requiring large inputs of heat to the digester together with close control of the digester temperature level. As previously discussed, the prior art, faced with these considerations, has utilized the methane produced by the anaerobic digestion process as heating fuel for the digester, to maintain constant elevated temperature even under extreme ambient temperature fluctuations. Such use of methane has proven effective in minimizing the large heating energy requirements of the process.
As an alternative to the foregoing methods, biodegradable sludge can be digested aerobically. Air has commonly been employed in practice as the oxidant for this purpose. It is known that aerobic digestion proceeds more rapidly at elevated temperatures. As temperature rises from 35.degree. C., the population of mesophilic microorganisms decline and thermophilic forms increase. The temperature range of 45.degree. C. to 75.degree. C. is often referred to as the thermophilic range where thermophils predominate and where most mesophils are extinct. Above this range, the thermophils decline, and at 90.degree. C., the system becomes essentially sterile. Because of the more rapid oxidation of sludge, thermophilic digestion achieves more complete removal of biodegradable volatile suspended solids than the same period of digestion at ambient temperature. A more stable residue is obtained which can be disposed of without nuisance. It is also established that thermophilic digestion can effectively reduce or eliminate pathogenic bacteria in the sludge, thereby avoiding the potential health hazard associated with its disposal.
When diffused air systems are used to supply oxygen for digestion, with the air being passed through the body of sludge in a digestion tank and freely vented to the atmosphere, the loss of heat from the sludge to the air being passed through the digester tends to become substantial in magnitude. As a result, aerobic digestion using air has heretofor typically involved digestion with mesophilic microorganisms. Air systems in general are not employed to carry out thermophilic digestion, unless a substantial level of heating energy is readily available for maintaining temperature of sludge in the digester in the thermophilic range. Such situation may for example exist if the digestion system is located in close physical proximity to a power generating plant which produces a large quantity of waste heat, so that such heat energy is in essence "free" for use in the digestion facility. Air contains only 21% oxygen and only about 5-10% of the oxygen content thereof is dissolved. As a result, a very large quantity of air must be used to supply the oxygen requirements, and the sensible heat of the "spent" air and the latent heat required to saturate the spent air with water vapor are substantial. As a result of these heat losses in air digestion, autothermal heat effects are generally minor, and very large quantities of external heat are needed to sustain temperatures at beneficial levels.
It is known that the heat losses in aerobic digestion can be greatly reduced by using oxygen-enriched gas rather than air. If the oxygen is utilized effectively, the amount of gas which must be fed to and vented from the digester is considerably smaller compared to air, because much or all of the nitrogen has been preliminarily removed. Heat losses due to sensible warmup of the gas and to water evaporation into the gas are decreased. These reductions in heat losses are sufficient for autothermal heat alone to sustain temperature at levels appreciably higher than ambient, so that the digestion zone is able to operate efficiently in the thermophilic temperature regime with little or no input of external heat to the process. Since thermophilic stabilization is much more rapid than mesophilic stabilization, the necessary residence time in the aerobic digestion zone is greatly reduced in the thermophilic mode. This in turn permits the use of smaller basins which further reduces heat losses to the surroundings. Because of the faster rate of oxidation of sludge, thermophilic aerobic digestion can achieve suitably high biodegradable volatile solids reduction, as for example, 80-90% reduction levels, in comparatively short sludge retention periods on the order of 3 to 10 days.
Despite its substantial attractiveness, thermophilic aerobic digestion has several associated disadvantages relative to anaerobic digestion. First, since the thermophilic aerobic digestion process is oxidative in character, the process produces a bio-oxidation reaction product gas containing carbon dioxide and water vapor which have no end use utility but rather are desirably vented to the atmosphere. By contrast, anaerobic digestion produces methane gas as a reaction by-product which may be exported from the treatment facility and is also useful as a fuel gas for satisfying the heating energy requirements associated with digestion at elevated temperatures. In addition, the aerobic digestion zone requires a much greater energy expenditure, for mixing and gas-sludge contacting, than is required in the anaerobic digestion system for mixing of the digester contents.
Accordingly, it is an object of the present invention to provide an improved process for digestion of sludge.
It is another object of the invention to provide a sludge digestion process employing aerobic digestion and anaerobic digestion at elevated temperature, in a manner which utilizes the advantages of each while minimizing their attendant disadvantages.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.